Method of making electrodes for quadrupole type mass spectrometers

ABSTRACT

A method of making electrodes for a quadrupole type mass spectrometer which comprises the steps of extruding glass into rods having a hyperboloidal surface and coating the hyperboloidal surface with a conductive film by sputtering or vapor deposition.

United States Patent [1 1 Wiley Feb. 19, 1974 [54] METHOD OF MAKING ELECTRODES FOR 3,492,215 1/1970 Conant 204/192 QUADRUPOLE TYPE MASS 1,751,587 3/1930 Loewe 117/227 1,717,712 6/1929 Loewe a a 1171227 SPECTROMETERS 3,376,449 4/1968 Harrison 117/227 [75] Inventor: William C. Wiley, Pittsford, N.Y. 3,557,440 H1971 Haberecht /2 X 2,923,624 2/1960 Hensler 117/118 X [73] Assignee: The Bendix Corporation, Southfield,

Mich.

Primary ExaminerRalph S. Kendall 22 F l F 22 i 1 1 ed eh 1971 Attorney, Agent, or Firm-Raymond .1. Eifler [21] App]. No.: 117,598

[52] US. Cl 117/213, 117/229, 204/192, 57 ABSTRACT 250/419, 65/60 [5 Cl- A method of making electrodes for a quadrupole Fleld of Search... 217, mass pectrometer comprises the steps of ex- 7/107; 204/192 298; 65/60 truding glass into rods having a hyperboloidal surface and coating the hyperboloidal surface with a conduc- [56] References Cited tive film by sputteringor vapor deposition.

UNITED STATES PATENTS 3,630,873 12/1971 Moore et a1. 204/192 6 Claims, 4 Drawing Figures PAIENIED FEB I 9 I974 FIGURE I FIGURE 3 FIGURE 2 FIGURE 4 WILLIAM C. WILEY INVENTOR. BY 1 fu ATTORNEY METHOD OF MAKING ELECTRODES FOR .QUADRUPOLE TYPE MASS SPECTROMETERS BACKGROUND OF THE INVENTION This invention relates to a mass filter of the type having a plurality of axially elongated electrodes arranged about a central axis for creating a time periodical electric field therebetween to separate ions having different mass-to-charge ratios. The invention is more particularly related to a method of making the elongated field forming electrodes.

Quadrupole analyzers or spectrometers have been described in the prior literature and particularly in U. S. Pat. No. 2,939,952. In such analyzers a controlled voltage is used in conjunction with a controlled frequency in performing a mass spectrum analysis. Such quadrupole analyzers may'be used in measuring the composition of chemical substances and are comprised basically of an ionizer, a quadrupole section, and an ion detector. The chemical substance which is to be analyzed is introduced into the ionizer as a vapor at a low pressure comprised of neutral atoms and molecules. A small proportion of the atoms or molecules which make up the chemical substance are ionized by electron bombardment and these ions are then accelerated and focused through an ion injection aperture into the quadrupole section in the form of an ion beam. The size of the aperture determines the cross-sectional size of the ion beam. Such an ion beam is then filtered in the quadrupole filter section of the analyzer, permitting only those ions within a specific range of charge-to-mass ratios to pass through the quadrupole-section. Those ions which are able to pass through the quadrupole section are then collected by the ion detector such as an electron multiplier.

The output current produced by the ion detector is a measure of the number of atoms or molecules in the ion beam which have a particular charge-to-mass ratio. The specific charge-to-mass ratio, which is detected, is determined by adjusting the frequency and magnitude of the voltages applied to the electrodes of the quadrupole section. Since a majority of the ions which are ionized are singly charged, the mass number of the detected atoms or molecules may be directly determined from the output current of the detector and the value of the voltages applied to the quadrupole electrodes.

The important parameters of a quadrupole mass analyzer are sensitivity, resolution, and contamination. Sensitivity is proportional to the number of ions that can be effectively employed by the analyzer per unit pressure in the analyzer. It is desirable to have the sensitivity as high a value as possible so that the signal to noise ratio is high as well as the speed of response. It is known that sensitivity of the analyzer increases as the ion beam current is increased. This increase in sensitivity occurs for the reason that the magnitude of the detected signal by the ion detector is directly proportional to the number of ions that enter the quadrupole section within a given length of time if it is assumedthat all Other parameters remain unchanged and that internal ion beam interactions are negligible.

The contamination of an analyzer is the collection on the quadrupole electrodes of undesirable substances which decrease the sensitivity and resolution of the analyzer. Normally, contamination accumulates on the electrodes'due to the bombardment of ions being rejected by the filtering action of the quadrupole filter.

section of the analyzer.- Excessive contamination occurs when more ions than can be effectively employed in the analyzer are injected through the ion injection aperture into the quadrupole section. In this manner, undesirable and rapid deterioration of analyzer performance occurs.

The resolution of the analyzer is defined as its ability to separate different masses that are being detected. The resolution of the analyzer is dependent upon many parameters, an important one of which is the shape and symmetrical arrangement of the electrodes.

. It is the shape of the electrodes that establishes the configuration of the electric field therebetween when a potential is applied to the electrodes. Should the shape of the electric field be distorted, ions that should pass through the filter are discharged on the electrodes and ions that should not pass through the filter are permitted through. The ideal shape for an electrode, as pointed out in U. S. Pat. No. 2,939,952 to Paul et al., is a hyperboloidally shaped electrode. However, due to the problem associated with obtaining uniform hyperboloidal surfaces, quadrupole electrodes have been fabricated from steel rods and machined smooth. For examples of metal rods used in quadrupoles see U. S. Pat. No. 3,371,205 to E. C. Berry; U. S. Pat. No. 3,129,327 to W. M. Brubaker; and US. Pat. No. 3,143,647 to K. Grunther et al. In these arrangements, cylindrical rod electrodes were used to approximate the field that would be established by hyperboloidal electrodes. As the development of quadrupole type mass filters progressed, the need for better resolution increased. However, present methods of fabricating electrodes have not resulted in a method of forming quadrupole electrodes that have a consistently uniform hyperboloidal surface which would result in an ideally shaped electric field and thereby improve resolution.

SUMMARY OF THE INVENTION To improve the shape of the electric field established by the electrodes of a nonmagnetic mass filter and thereby improve the resolution of the filter, especially a quadrupole type filter, the electrodes of the filter are comprised of extruded glass rods having a thin conductive film thereon.

The invention is characterized by a method of making the electrodes for a mass filter which comprises: extruding glass into rods having a hyperboloidal shape and sputtering or vapor depositing a conductive film thereon which is of a predetermined thickness or thickness extending a predetermined length along the surface of the glass. In one preferred method of the invention, glass is extruded to the preferred hyperboloidal shape, the surface is then treated to obtain a resistive conducting surface, a metal film is deposited thereon by sputter coating techniques to achieve a desired thickness and the glass is then cut into predetermined lengths for assembly into a mass filter.

Accordingly, it is an object of this invention to fabricate electrodes by extruding glass to the preferred electrode shape and then sputter coating or vapor depositing a thin conductive film on the surface of the extruded electrode.

It is another object of this invention to improve the resolution of a quadrupole 'type mass filter.

It is a still further object of this invention to have an electric-field of improved shape by utilizing extruded glass electrodes having a hyperboloidal surface in a quadrupole type mass spectrometer.

It is still another object of this invention to improve the overall operation and performance of a quadrupole mass analyzer.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification. Y

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a quadrupole type mass filter utilizing electrodes that embody the principles of-the invention; I

FIGS. 2 and 3 show end views of extruded glass electrodes.

FIG. 4 is a plan view of an extruded glass electrode.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a quadrupole type filter which utilizes electrodes 1 fabricated from the process disclosed herein.

FIGS. 2 and 3 illustrate electrodes fabricated by the extrusion and sputtering process of this invention which have cross sections that include a hyperboloidal surface Extrusion is a squeezing process wherein glass is forced to flow rapidly through the orifice of a die. By extruding glass to form an electrode, the electrode may take any complicated configuration defined by the shape of the orifice. Therefore, to achieve a particular shape, expensive machining, which is required when fabricating metal electrodes, is not required.

FIG. 4 shows a plan view of an extruded glass rod lv that has had a thin metallic film 3 applied thereto by sputtering. The metallic film 3 has a sheet resistance of less than 100 ohms per square.

Sputtering is the rathergraphic term describing the process of disintegrating a solid surface by bombarding it with tiny particles, usually'ions accelerated toward the surface by a high voltage. The momentum of the impacting ions is transferred to the surface atoms, ejecting them with farily high velocities of their own. The ejected atoms can thus'be deposited on a substrate, providing a method of constructing thin films atom by atom. Since sputtering is a nonevaporative process, high-melting-point materials like tantalum and tungsten, and even ceramics, can be deposited. This makes .the technique'useful in microelectronics and other areas where films of such materials are desired. Sputtering has many noncoating uses too, ranging from surface cleaning and vacuum pumping to studying the surface of the moon. For examples of sputtering apparatus see U. S. Pat. No. 3,305,473 to R. M. Moseson and U.S. Pat. No. 3,393,142 to R. M. Moseson. As previously mentioned, the metallic film on the glass electrode also maybe applied by vapor'depositionor a combination of sputtering and vapor deposition (See U.S. Pat. No. 3,492,215 to LA. Conant).

In addition to the metallic film 3 applied to the extruded glass rod 1, the'glass rod 1 may be treated to produce a surface 5 that has a sheet resistance over a wide range 10 to 10 ohms per square). For instance, glass with a high lead content can be rendered active (conductive, with a resistance per. square previously mentioned) by subjecting the glass to a high temperature in a hydrogen atmosphere, This additional resistive surface coating 5, having a different sheet resistance per square than the conductive coating 3, is used in combination with the conductive coating 3 to achieve a predetermined electric field gradient at the end portions of the electrodes when a voltage is applied to the conductive coating 3 and the resistive coating 5.

A PREFERRED METHOD To produce rods of complex cross-sections, inexpensively and with high accuracy, glass is extruded to a predetermined shape. After the glass is extruded, it is cut into working lengths and placed in a sputtering apparatus where a metallic film of predetermined thickness and resistance is applied over a predetermined portion of the glass rod. Altemately, the metallic coating may be applied by vapor deposition or a combined vapor deposition and sputtering techniques. Also, before the metal coating is applied, the glass rods may be treated to make the surface of the glass a resistive conducto'r. j

Once the glass rods are coated with the thin metallic film, they are electrodes ready for assembly into a mass filter.

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, although the invention has been illustrated as-useful in a quadrupole type mass filter, it is also useful in duopole, monopole, or similar type filters. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is:

l. A process of making electrodes from a high lead content glass which comprises the steps of:

extruding said glass to a predetermined elongated configuration that includes a hyperboloidal surface;

treating said extruded glass to obtain a resistive surface having a sheet resistance in the range of 10 to 10 ohms per square by subjecting the glass to a high temperature in a hydrogen atmosphere; and sputtering a conductive filmhaving a sheet resistance of less than ohms per square on said hyperboloidal portion of said extruded glass surface.

2. A process as recited in claim 1 wherein said conductive film is comprised of metal.

3. A process of making electrodes from glass, comprising the steps of:

extruding said glass to a predetermined configuration; that includes at least one hyperboloidal surface; and

sputtering a conductive material on said hyperboloidal surface to obtain a substantially uniform thickness of less than 15 mm.

4. A process as recited in claim 3 wherein said conductive material is metal.

5. A process of making electrodes from glass comprising the steps of:

extruding said glass into an elongated member having a hyperboloidal surface on at least a portion of said member; and

sputtering at least a portion of said hyperboloidal surface with a conductive material to obtain a continuous conductive coating varying in thickness in a predetermined manner.

6. A process as recited in claim 5 wherein said coating is comprised of metal. 

2. A process as recited in claim 1 wherein said conductive film is comprised of metal.
 3. A process of making electrodes from glass, comprising the steps of: extruding said glass to a predetermined configuration; that includes at least one hyperboloidal surface; and sputtering a conductive material on said hyperboloidal surface to obtain a substantially uniform thickness of less than 15 mm.
 4. A process as recited in claim 3 wherein said conductive material is metal.
 5. A process of making electrodes from glass comprising the steps of: extruding said glass into an elongated member having a hyperboloidal surface on at least a portion of said member; and sputtering at least a portion of said hyperboloidal surface with a conductive material to obtain a continuous conductive coating varying in thickness in a predetermined manner.
 6. A process as recited in claim 5 wherein said coating is comprised of metal. 